Radio frequency (RF) power transistors are used e.g. for mobile communication devices in the frequency range from 900 MHz to 2 GHz. Failure of RF power transistors can occur due to a mismatch of the load impedance (e.g. an antenna) and the output impedance of the transistor. Due to the impedance mismatch the voltage between base and collector of the RF transistor may increase above the level where breakdown occurs and the transistor becomes damaged. A protection diode connected between collector and emitter will prevent breakdown of the transistor by providing a lower breakdown voltage for the diode. Such a device is known from U.S. Pat. No. 5,751,052, which shows a transistor integrated with a Zener diode as protection diode. In FIG. 2 of U.S. Pat. No. 5,751,052 a cross-sectional view of a device comprising a transistor and a protection Zener diode is shown. During the manufacturing of a device according to this prior art, the construction of the Zener diode requires additional processing steps. As known to persons skilled in the art of semiconductor technology, such steps comprise the formation of precisely defined lateral diffusion zones for which the respective depth must also be defined precisely.
Therefore, the formation of a Zener diode disadvantageously contributes to the manufacturing time and costs of the device. Moreover, the formation of a Zener diode may be prohibited in some types of power transistor device due to restrictions imposed by the technology. For example, in a double poly-Si process for bipolar devices as known in the art, formation of a Zener diode connected to a RF power transistor is not feasible.